1. Field of the Invention
The present invention relates to an overhead projector with a spatial light modulator, and, in particular, to an overhead projector with a spatial light modulator in which a superior contrast between light and shade is obtained.
2. Description of the Related Art
2.1. Previously Proposed Art
As is well known, an overhead projector has been conventionally utilized to project only a manuscript drawn in a film type of transparent manuscript paper onto a screen. However, an overhead projector utilizing a spatial light modulator has been laid open to public inspection under Japanese Patent Provisional Publication No. 17615/1991 (HEI 3-17615). The overhead projector with a spatial light modulator is utilized to project a manuscript drawn in an opaque film.
In detail, as shown in FIG. 1, a manuscript drawn in an opaque film 11 is illuminated with writing light Lw before the writing light Lw is reflected by the opaque film 11. In this case, because large pieces of writing light Lw are respectively absorbed according to the manuscript to for the image of the manuscript, the intensities of the large pieces of writing light Lw are varied according to the manuscript. Thereafter, one side of a light valve 12 with a liquid crystal are irradiated with the writing light Lw reflected by the opaque film 11 to write the image of the manuscript in the light valve 12. Here, the light value 12 functions as the spatial light modulator. That is, the light valve 12 consists of a large number of regions, and each region is irradiated with prescribed pieces of writing light Lw.
Thereafter, the other side of the light valve 12 is irradiated with visible reading light Lr to read out the image of the manuscript from the light valve 12. That is, large pieces of visible reading light Lr are modulated in the light valve 12 for each region according to the manuscript written in the light valve 12, and the visible reading light Lr reflected by the light valve 12 is projected onto a screen 13.
The light valve 12 with a liquid crystal comprises layers of a photoconductive layer 14 for receiving the writing light Lw reflected by the opaque film 11, a reflecting layer 15 for reflecting the visible reading light Lr, and a liquid crystal layer 16 for modulating the visible reading light Lr, in that order. In addition, the photoconductive layer 14 consists of a large number of regions, the reflecting layer 15 consists of a large number of regions, and the liquid crystal layer 16 consists of a large number of regions. Each of the regions of the photoconductive layer 14, each of the regions of the reflecting layer 15 and each of the regions of the liquid crystal layer 16 are in alignment with one another.
In the above configuration, when the photoconductive layer 14 is irradiated with the writing light Lw, an electrical resistivity of the photoconductive layer 14 is lowered for each region of the photoconductive layer 14 in proportion to the intensity of the writing light Lw. That is, the manuscript drawn in the opaque film 11 is written in the photoconductive layer 14 as image information. Therefore, in cases where a prescribed electric potential difference is applied between the photoconductive layer 14 and the liquid crystal layer 16, voltages applied to the regions of the liquid crystal layer 16 are increased in proportion to electrical resistivities of the regions of the photoconductive layer 14. Therefore, the image information of the manuscript written in the photoconductive layer 14 is transferred to the liquid crystal layer 16 as a voltage distribution of the liquid crystal layer 16.
Thereafter, when the liquid crystal layer 16 is irradiated with the visible reading light Lr, the visible reading light Lr is modulated in the liquid crystal layer 16 according to the voltage distribution of the liquid crystal layer 16. That is, large pieces of visible reading light Lr are polarized in the liquid crystal layer 16 according to a double refraction phenomenon for each region of the liquid crystal layer 16. Thereafter, the visible reading light Lr is reflected by the reflecting layer 15 to pass through the liquid crystal layer 16 once more. Thereafter, the intensities of the large piece of visible reading light Lr are varied by a polarizer for each region because the light Lr is poralized in the liquid crystal layer 16. This means that the image information of the manuscript transferred in the liquid crystal layer 16 is read out by the visible reading light Lr because an intensity distribution of the large pieces of visible reading light Lr depends on the manuscript. Thereafter, the image information read out by the visible reading light Lr is enlarged by a prescribed optical system 17 before the image information is projected onto the screen 13.
Accordingly, the manuscript drawn in the opaque film 11 can be enlarged and be reproduced onto the screen 13.
2.2 PROBLEMS TO BE SOLVED BY THE INVENTION:
However, in cases where a part of the visible reading light Lr penetrates the reflecting layer 15 of the spatial light modulator such as the light valve 12 with a liquid crystal, the photoconductive layer 14 is optically reacted by the part of the visible reading light Lr so that the electrical resistivity of the photoconductive layer 14 receiving the visible reading light Lr is varied. Therefore, the image of the manuscript projected onto the screen 13 becomes unclear. In other words, the contrast of the image projected onto the screen 13 deteriorates.
Moreover, in general, the liquid crystal layer 16 is easily irradiated with the visible reading light Lr with a high intensity to project a bright image of the manuscript onto the screen 13. Therefore, the quantity of the visible reading light Lr penetrating the reflecting layer 15 is further increased. As a result, the contrast of the image projected onto the screen 13 increasingly deteriorates.
Therefore, the overhead projector with the spatial light modulator is further provided with a shading layer 18 between the photoconductive layer 14 and the reflecting layer 15 to improve the contrast of the image by shading the visible reading light Lr penetrating the reflecting layer 15. However, the resolution of the image projected onto the screen 13 deteriorates because the shading layer 18 prevents the image of the manuscript from being transferred from the photoconductive layer 14 to the liquid crystal layer 16. The deterioration of the resolution is increased as the thickness of the shading layer 18 becomes thick. Therefore, it can be considered that shading properties of the shading layer 18 are raised to make the thickness of the shading layer 18 thin. However, the electrical impedance of the shading layer 18 is lowered as the thickness of the shading layer 18 becomes thin. In this case, an electric field generated in one region is spread over the other regions. Therefore, the resolution of the image deteriorates.
In addition, the liquid crystal layer 16 of the light valve 12 is generally formed by a liquid crystal with a homogeneous orientation because the homogeneously oriented liquid crystal layer 16 functions at a low electric voltage and is easy to deal with. However, the intensity of the reading light Lr modulated in the liquid crystal layer 16 largely depends on the wavelength of the reading light Lr. The reason why the intensity of the modulated reading light Lr largely depends on the light wavelength is described with reference to FIGS. 2, 3.
As shown in FIG. 2, a double refractive index .DELTA.n of a liquid crystal depends on a wavelength .lambda. of light. In cases where the liquid crystal is sandwiched between a pair of polarizers of which the polarizing directions are crossed each other, an intensity I.sub.o of the light is reduced to an intensity I.sub.t after the light is transmitted through one polarizer, the liquid crystal, and the other polarizer in that order. That is, following equations are satisfied EQU I.sub.t =I.sub.o * sin.sup.2 (.pi.R/.lambda.) EQU R=d*.DELTA.n(.lambda.)* cos.sup.2 .phi.(V),
where the symbol d represents the thickness of the liquid crystal and the symbol .phi.(V) represents the inclined angle of molecules in the liquid crystal. The value of the angle .phi.(V) depends on a voltage V applied to the liquid crystal.
In this case, the condition that the intensity I.sub.t of the transmitted light is at a maximum is as follows. EQU cos.sup.2 .phi.(V.sub.p)=.lambda..sub.p /{d*.DELTA.n(.lambda..sub.p)}*(m+1/2) m=0, 1, 2,.
That is, when the voltage V.sub.p is applied to the liquid crystal, the intensity I.sub.t of the transmitted light with the wavelength .lambda..sub.p equals I.sub.o.
When the liquid crystal is irradiated with large pieces of light with a waveband, the intensities I.sub.t of the large pieces of transmitted light are varied for each light wavelength as shown in FIG. 3. That is, the transmitted light intensities I.sub.t are reduced as the value m becomes high.
In cases where the electric potential V.sub.p is applied to the homogeneously oriented liquid crystal layer 16 for practical use, the value m equals 3 or 4 as is well known. Therefore, the intensity I.sub.t of the transmitted light is largely varied as compared with the case that the value m equals 0 or 1.
Therefore, the contrast of the image deteriorates unless the waveband of the reading light is extremely narrowed. Moreover, in cases where the waveband of the reading light is narrowed to avoid the deterioration of the contrast, the luminance of the image projected onto the screen 13 is decreased because the intensity of the light transmitted to the liquid crystal layer 16 is lowered. Specially, it is very difficult to obtain a colored image of a colorful subject at a high luminance onto the screen 13 while maintaining the reproductivity of the color.
In addition, the light valve 12 with a liquid crystal is merely applied to an overhead projector without any modification. Therefore, it is not considered how to conveniently utilize the light valve 12. For example, a three-dimensional subject cannot be projected onto the screen 13.